The subject of the invention is a system for electrical energy exchange between at least one motor-generator element and at least one element for storing a DC voltage and applies, in particular, to a vehicle having a so-called “hybrid” propulsion system.
Increasingly severe constraints in terms of the compromise between performance, consumption and emission are leading to the development of so-called “hybrid” vehicles which seem to afford an efficacious solution to all the problems raised.
In a general manner, a hybrid-propulsion vehicle comprises a main engine, for example an internal combustion engine, which propels the driving wheels through a mechanical transmission system, and an electrical power bypass system comprising at least one electric machine wired to an energy storage element such as a battery and able to operate either as a motor providing a torque that is substituted for or added to the torque provided by the main engine, or as a generator providing a DC voltage for recharging the battery from the torque provided by the driving wheels in the case of braking or slowing.
Various hybrid vehicle types are known. Document WO 2005/081 387, for example, describes a system comprising two electric machines able to operate as motor or generator.
In a so-called electrical power bypass operating mode, at least part of the mechanical power provided by the main engine serves to propel a first machine operating as a generator so as to provide electrical power applied to the second machine which operates as a motor applying to the wheels, by way of the transmission system, a torque which depends on the rotation speed of the electric motor. Such a system makes it possible, in particular, to produce a continuous variation of the transmission ratio electrically.
FIG. 1 is a basic diagram of such a vehicle with electrical power bypass comprising an engine M, driving a shaft A connected to the driving wheels R by a mechanical transmission T and, optionally, a differential D and an electrical power bypass system S comprising two motor-generator electric machines MG1, MG2 operating under three-phase current and connected electrically to a battery B, each by way of a DC/AC converter C1, C2.
Indeed, in a known manner, described in the document WO 2005/081 387, the voltage provided, during discharge by the battery B or applied to the latter during charging, is controlled by a DC/DC converter wired to the two terminals, respectively positive and negative, of the battery B, and connected to the two DC/AC converters C1, C2 by a circuit 2 called a “bus” having two branches, respectively positive 21 and negative 21′, each wired up, from a nodal point 22, 22′, to the two DC terminals, respectively 31, 31′ of the DC/AC converter C1 and 32, 32′ of the converter C2.
A filtering capacitor 3 is wired in parallel between the two positive 21 and negative 21′ buses.
The construction and the operation of the various circuits of an electrical system such as this is described in detail in document WO 2005/081 387. In particular, each DC/AC converter comprises three arms wired in parallel with the DC terminals of the converter and corresponding respectively to the three phases of the motor MG1, MG2, each arm comprising two transistors each arranged with a diode antiparallel fashion.
Likewise, in such a layout, the DC/DC converter 1 comprises, in the manner represented schematically in FIG. 2, an inductor 11 having an input 11a wired to the positive terminal of the battery B and an output 11b wired to an intermediate neutral point 12 connected, respectively, to the positive bus 21 and to the negative bus 21′ by two circuits in series 13, 13′ each comprising a diode 14, 14′ arranged in antiparallel fashion with a power transistor 15, 15′.
As shown by FIG. 2, the diodes 14, 14′ allow the passage of the current between the two branches 21, 21′, and the neutral point 12, to which the output of the inductor 11 is wired, is connected on one side to the emitter of the transistor 15 whose collector is connected to the positive bus 21 and on the other to the collector of the transistor 13′ whose emitter is connected to the negative bus 21′.
In a known manner, as shown schematically by FIG. 2, to increase the admissible power, the converter 1 can comprise one or more other arms in parallel, with interleaved control, in the manner represented dashed in FIG. 2.
In a general manner, such a DC/DC converter makes it possible to boost the voltage at the terminals 31, 31′ of the filtering capacitor 3, with respect to the voltage delivered at the terminals of the battery B. It makes it possible, on the one hand, to control the energy exchanges between the battery B and the electric machines MG1, MG2 in so-called “boost” motor mode or in recuperative braking mode and, on the other hand, to maintain the voltage of the filtering capacitor 3, that is to say between the positive bus 21 and negative bus 21′, at an optimal value having regard to the conditions of forward progress of the vehicle which correspond to a torque and a certain rotation speed of the electric machines MG1, MG2.
Such a system for energy exchange between a storage element such as a battery and at least one motor-generator electric machine, comprising a DC/DC converter, a filtering capacitor and a DC/AC converter, is well known and had also been described in document EP 1 138 539. In this case, the electric machine served, as motor, to start the engine and, as generator, to recharge the battery, such a system thus making it possible to avoid employing an alternator.
Moreover, other layouts are possible for producing a DC/DC converter making it possible to boost the voltage between the two buses, positive and negative, and to control said voltage at an optimal value. For example, such a converter can be used as a supplement to an alternator in the manner described in the document FR-A-2 858 484 which gives several exemplary embodiments of a DC/DC converter comprising, in a general manner, an inductor having an input wired to the positive terminal of the battery and an output wired to an intermediate point connected to the negative bus and to the positive bus by two circuits in series, respectively a circuit generating a pulsed current and a voltage-raising circuit.
However, subsequently in the text, reference will essentially be made to the type of DC/DC converter described in the document WO 2005/081 387 and represented schematically in FIG. 2, comprising an inductor and two power transistors of the type with insulated gate termed an “Insulated Gate Bipolar Transistor” (IGBT).
As indicated above, such a converter makes it possible to maintain the voltage across the terminals of the filtering capacitor at an optimal value which depends on the conditions of forward progress of the vehicle, that is to say the torque/speed operating point of the electric machine or of the two machines in the case of document WO 2005/081 387. For this purpose, this document describes in detail how a control unit receiving information representative of the torque requested from each of the two motors and of their rotation speed, determines, at each instant, the requested optimal voltage between the terminals of the converter and orders the switching of the various circuits so as to obtain and maintain this optimal voltage.
In particular, it is known that, to determine the operating conditions of an electric machine, it is possible to plot a chart of the type represented in FIG. 3 indicating the torque as a function of the rotation speed and it is known that the envelope curve depends on the DC voltage across the terminals of an inverter. For example, represented in FIG. 3 are the envelope curves in motor mode corresponding respectively to DC voltages V1, V2, V3 with                V1>V2>V3        
and it is apparent that the accessible torque at high speed is all the larger the higher the voltage.
This is why it is beneficial to have a stable filter voltage at an optimal value for the cost/performance ratio of a machine-inverter assembly. In the case of a hybrid vehicle represented in FIG. 1, the use of the DC/DC converter between the storage element B and the filter 3 shared by the inverters C1, C2 makes it possible, in the manner described in document WO 2005/081 387, to control the energy exchanges and to maintain the voltage of the filter at its optimal value.
However, the object of a hybrid vehicle is essentially to decrease fuel consumption and, consequently, pollution, by driving the wheels in an electrical manner whenever possible, on the basis of the energy stored in the battery or else, in a power bypass system, to produce electrically a continuous variation of the transmission ratio making it possible to choose, at each instant, an optimal ratio.
Now, in the conventional diagram of FIG. 1, the DC/DC converter 1 which is inserted into the efficiency chain decreases, through its own losses, the autonomous range under electric propulsion.
Moreover, the whole of the traction electrical power passes through the DC/DC converter, thereby requiring, for relatively high speeds, above 50 km/h for example, significant power and, therefore, an expensive, heavy and bulky converter.
This is why, by envisaging a large storage capacitor, the autonomous range would be degraded, and likewise for the mass and the cost of the assembly.